Dental x-rays are typically taken with a film that is placed in the patient's mouth. The film is exposed through the teeth by an x-ray source that resides outside the patient's head. While this method has been in use for many years, it has its disadvantages. First, the patient is exposed to a significant dose of x-rays. This dose is accumulative over the patient's lifetime. Second, the time, cost, and equipment needed to process the film increases the cost of the dental examination. Third, the chemicals utilized in processing the film pose a disposal problem.
These problems have led to several attempts to replace the film component of the traditional x-ray examination with a solid-state sensor that is placed in the patient's mouth to record the x-ray image. In such systems, a layer of scintillation material is used to convert the x-rays to visible light. The visible light is then imaged onto a solid-state imaging array. Since solid-state x-ray sensors of this type are significantly more sensitive to x-rays than the films utilized today, the x-ray dosage can be reduced by typically a factor of 10. In addition, the sensor is re-used, and hence, the cost and disposal problems associated with the conventional x-ray system are avoided. Finally, since the image is in digital form, systems based on solid-state sensors are easily adapted to paperless office systems.
Ideally, the sensor that is placed within the patient's mouth has smooth edges to avoid injury to the patient's gums during the examination. Accordingly, sensors that have chamfered corners are preferred. Unfortunately, conventional solid-state imaging sensors have a shape that is predominately rectangular. This shape is the result of the need to utilize the silicon substrate as efficiently as possible and the conventional processes in which the dies are eventually separated from a wafer by sawing the wafer into rectangular pieces. Hence, to provide a die in which the corners are removed, the die must be made significantly larger to allow the sensor to be located in the interior of the die and still provide sufficient space to cut-off the corners without harming the sensor region.
Sensors having chamfered corners based on CCD imaging arrays are known to the art. For example, U.S. Pat. No. 5,510,623, teaches a CCD x-ray dental sensor with chamfered corners. However, sensors based on CCD arrays are less than ideal for intra-oral x-ray sensors. In particular, CCD sensors are more sensitive to degradation by x-rays than CMOS image sensors. Hence, an x-ray shielding layer is needed between the scintillation layer and CCD array to protect the CCD array from x-rays that are not converted in the scintillation layer. This layer degrades the image. In addition, CCD arrays require relatively high voltages to operate and are more expensive to fabricate than CMOS sensors. Since cost is an important factor in achieving acceptance of a new imaging system by the dental community, these additional costs can be a serious barrier to the introduction of solid-state imaging systems. Hence, it would be advantageous to have an x-ray imaging system based on a CMOS image sensor.